The present invention relates to novel adenine derivatives represented by the following formula and agriculturally acceptable acid addition salts thereof, to compositions containing such compounds and to their use as agents for increasing yield of crops. ##STR2## (wherein n is an integer of 2-4)
Hitherto, many substances have been found for adjustment of growth of plants such as crops, fruits and vegetables. Among them, those which belong to a group of substances generically called cytokinin-active substances are known to have many useful physiological activities. Representative compounds are zeatin, kinetin and 6-benzylaminopurine (hereinafter referred to as benzyladenine).
Such cytokinin-active substances have been known to have various useful properties for plants such as the control of growth by cell division in callus and intact plants, the promotion of buds and flower formation, the stimulation of seed germination, the overcoming of dormant phase of seeds, storage organs and buds, the promotion of sideshoot formation, and the retardation of the ageing processes in plants.
Several substances which show cytokinin activity have occur naturally, but such naturally occurring substances are available only in limited quantities and are difficult to obtain. In addition to those natural cytokinins, various synthetic adenine derivatives have been shown to have cytokinin activity. However, their practical use is quite limited. One of the reasons for such limitation is that both natural and synthetic cytokinins are poor in water-solubility and absorption into plants with the result that translocation to the organs is insufficient.
Previously, various studies have been made for increasing the yield of cereal crops such as waterfield rice plants and beans, but few of them have been practically applied in spite of many efforts.
It is an object of the present invention to provide active ingredients which are higher in water-solubility than conventional cytokinin-active substances, rich in physiological activity for practical use and not difficult to manufacture.
It has been known that cytokinins such as kinetin, zeatin, benzyladenine and thidiazuron have activities such as the control of growth by cell division in callus and intact plants, the differentiation of plant tissues, the stimulation of seed germination and senescence retardation. Recently, an attempt has been made to increase yield of cereal crops such as waterfield rice plants with the above-mentioned cytokinins. For example, it is reported in "Japanese Journal of Crop Science", Vol. 51, Extra issue 1, 1982, page 147-148 that zeatin and abscisic acid increase the ripening rate and thousand-kernel weight of husked rice according to the experiments of direct injection of these plant hormones into ears of rice plant. More recently, Japanese Patent Kokai No. 63-181933 discloses that yield of gramineous crops can be increased by treating ears with benzyladenine, thidiazuron, kinetin or zeatin.
Natural cytokins such as zeatin and ribosylzeatin are difficult of manufacture. Benzyladenine kinetin and thidiazuron can be synthesized with relative ease, but have low water-solubility and manifest poor absorption into plants and translocation to other organs. Thus, these are not efficient for practical use.
As a result of extensive and intensive research conducted by the inventors in view of the problems cited above, it has been found that adenine derivatives having a side chain represented by the following formula (A) substituted on the N.sup.6 nitrogen atom of adenine and salts thereof are effective for attaining the above object. The present invention is based on such new finding. ##STR3##
It has also been found that among these adenine derivatives and salts thereof, N.sup.6 -[2-(N-methoxy-N-methylamino) ethyl]adenine and its salts exhibit especially strong cytokinin activity.
The inventors have studied the activity of N.sup.6 -[2-(N-methyoxy-N-methylamino)-ethyl]adenine and salts thereof on plants and have found that these compounds not only have strong cytokinin activity but also increase the yield of agricultural crops when applied to leaves and stems of leguminous plants and cereal plants such as gramineous plants.
The present invention is based on these new findings.